A recording system utilizing color formation reaction between a substantially colorless electron-donating dye (hereinafter referred to as a color former) and an electron-accepting compound capable of developing a color upon contact with the color former (hereinafter referred to as a color developer) has hitherto been well known. Such a recording system has been embodied as, for example, pressure-sensitive copying paper, heat-sensitive recording paper, electrothermo-recording paper, and the like and is described, e.g., in U.S. Pat. Nos. 2,712,507, 2,730,456, 2,730,457, 3,418,250, 3,432,327, 3,981,821, 3,993,831, 3,996,156, 3,996,405, 4,000,087, etc.
These conventional recording materials include a combination of an upper sheet comprising a support having coated thereon a microcapsule layer containing microcapsules of oil droplets comprising a color former in an appropriate solvent, a lower sheet comprising another support having coated thereon a color developer layer, and, if desired, an intermediate sheet comprising a support having coated a microcapsule layer on one side thereof and a color developer layer on another side thereof; a single sheet containing the color former-containing mecrocapsules and the color developer as coated on the same side of a support; and a single sheet comprising a support containing either one of the color former-containing microcapsules and the color developer, with another being coated thereon; and the like.
It has been proposed as described in U.S. Pat. Nos. 4,436,920 and 4,390,616 to use a diarylaminofluoran derivative as a color former in these recording materials, with which color images having remarkably excellent light-fastness can be provided.
However, this color former has a disadvantage of poor solubility in solvents used for the synthesis thereof or solvents used for encapsulation thereof.